An increasingly important utility for transparent polyester film has been solar control coatings for exterior windows, doors, skylights, building walls, such as in skyscrapers, and the like. Radiation from sunlight can penetrate uncoated windows and affect the articles inside such as exposed fabric and furniture. Over prolonged periods colors of articles exposed to sunlight can fade and may partially disintegrate. Coating bare window materials with solar transparent polyester control films is a technique for reducing the adverse effects of sunlight exposure. Transparent polyester films may used outside in a variety of other applications as well. For example, the films may be used in flexible photovoltaic film, reflection film for signage, and display film applications. These outside applications typically require the polyester films to be both UV and hydrolysis resistant.
Some transparent biaxially oriented polyester films are known in the art. For example, U.S. Pat. Nos. 6,709,740 and 6,706,387 disclose ultra low haze, co-extruded, thermoplastic polymer films with easy handling prepared by co-extruding a blend of polyethylene terephthalate with organic and inorganic fillers. However, these patents do not disclose films that are UV and hydrolysis resistant.
U.S. Pat. No. 6,593,406 discloses polyester films with enhanced UV stabilization properties. However, this patent does not disclose films that are resistant to hydrolysis.
U.S. Pat. No. 6,855,758 discloses transparent polyester films with UV and hydrolysis resistance made from PET resin with an intrinsic viscosity (IV) of 0.658 or 0.632. The films in this patent possess some degree of transparency, UV resistance and hydrolysis resistance. However, in order to achieve the required hydrolysis resistance, the hydrolysis stabilizers need to be used at higher than 1 mass percent. This high percentage of hydrolysis stabilizers would result in an excessive generation of organic volatiles due to their thermal sensitivity.
U.S. Pat. Nos. 6,794,432, 7,138,176, and 7,157,132 disclose transparent, UV resistant polyester films that contain a flame-retardant. A hydrolysis stabilizer is added to prevent the flame retardant from increasing the film's susceptibility to hydrolysis not to provide additional hydrolysis resistance. In addition, the haze of the films in the examples is greater than 2%.
Accordingly, a need exists for a polymeric transparent film for outdoor applications that has superior performance characteristics than prior art films and can cost effectively produced and easily applied to end use applications. It is desirable for the film to be weather resistant, i.e., resistant to hydrolysis under hot and moist conditions. It is also desirable for the film to employ only small amounts of a hydrolysis stabilizer to achieve good hydrolysis resistance. Finally, it is desirable for the film to effectively reduce the transmission of ultraviolet light, and remain optically clear even after long exposure to sunlight.